1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method, and particularly, to a print data generating configuration that enables an image print to have a tolerance for image quality degradation against variations of print characteristics between printing elements of a print head, a fluctuation in scanning of the print head, a conveying error of a print medium and the like.
2. Description of the Related Art
As an example of a printing system using a print head provided with a plurality of printing elements, there is known an inkjet print system which ejects ink from an ejection opening as the printing element to form dots on a print medium. Such an inkjet printing apparatus is classified into a full line type and a serial type depending particularly on a difference in construction of the print head.
The full line type printing apparatus is provided with the print head which includes printing elements arranged over a range corresponding to a width of the print medium conveyed and is used in a fixed state at printing. The print medium is conveyed in a direction, which intersects the array direction of the printing elements, relative to the print head used in the fixed state and ink is ejected to the print medium in a predetermined frequency from the print head to form an image. Such a full line type printing apparatus can form the image at a relatively high speed and is suitable for office use. On the other hand, in the serial type printing apparatus, a print head scans a print medium, ink is ejected thereon at a predetermined frequency during the scanning, and a conveying operation conveying the print medium in a direction intersecting with the scanning direction of the print head is performed for each scan to form an image. Such a serial type printing apparatus can be manufactured in a relatively small size and at low costs and is suitable for personal use.
In any of these full line type and serial type printing apparatuses, a plurality of printing elements arranged in the print head contain a certain degree of variations in the manufacturing process. These variations appear as variations of ejection characteristics such as an ejection amount or an ejection direction of ink to produce irregular shapes of dots formed on the print medium, as a result possibly creating uneven density or stripes on an image.
For overcoming this problem, a so-called multi-pass printing system is employed in the serial type inkjet printing apparatus, for example. In the multi-pass printing, pixels to which the print head can perform printing in one-time printing main scan are distributed to a plurality of scans of the print head between which a conveying operation of the print medium is performed so as to make different printing elements used in the plurality of scans for performing the printing operation. This allows the variations in ejection characteristics in the plurality of the printing elements to be dispersed into the plurality of scans for completing the image, enabling the uneven density to be indistinctive. This multi-pass print system can be also applied to the full line type printing apparatus.
As shown in FIG. 1, two lines of printing elements in regard to ink of the same color are arranged in a conveying direction of the print medium, thereby enabling the dot line formed in the conveying direction to be shared and printed by the two printing element lines. As a result, the variation of the printing elements in one printing element line is dispersed into ½, enabling the uneven density due to the variation to be indistinctive.
In a case of performing the multi-pass printing, print data of the image are distributed into plural times of printing scans or a plurality of print heads (printing element lines) for completing the image. Conventionally most of the times this distribution is carried out by using a mask pattern in which a pixel (“1”) permitting printing a dot and a pixel (“0”) not permitting printing a dot are in advance defined corresponding to an individual printing element.
FIG. 2 is a diagram showing an example of a mask pattern used in a multi-pass print for completing a printing by two times of scans (hereinafter, also called “pass”) in the serial system. In FIG. 2, black areas each show a pixel (“1”) permitting printing of a dot and white areas each show a pixel (“0”) not permitting printing of a dot, and number 501 denotes a mask pattern used in a scan of first pass and number 502 denotes a mask pattern used in a scan of second pass. The pattern 501 and the pattern 502 are complementary with each other in regard to print permitting pixels (or print non-permitting pixels), and therefore, dots constituting an image to be completed are formed in any one of the first pass and second pass. Specifically, in regard to print data of the image to be completed, a logical product is carried out for each pixel between image data to be completed and the above patterns and thus the result becomes binary data according to which respective printing elements actually executes printing in each pass.
However, an arrangement of the print data (“1”) in the pixels according to which printing is performed varies depending on the image to be printed. Therefore, it is difficult to always evenly distribute such a print data to the plurality of scans or plurality of printing element lines by using a mask pattern in which a pattern of the print permitting pixels is previously defined. Thus, a particular scan or a particular printing element line may print a high ratio of dots, and as a result, the ejection characteristic of the particular scan or of the particular printing element line appears in the image to decrease the original advantage of the multi-pass printing. Accordingly, in the multi-pass printing, how equally and evenly the print data are distributed into the plural scans or the plural printing element lines is one of important issues.
For example, Japanese Patent Laid-Open No. H07-052390 (1995) describes a method of producing a mask pattern in which print permitting pixels and print non-permitting pixels are arranged at a random. By using this random mask pattern, the print data can be expected to be distributed substantially equally to the plural scans and the plural printing element lines even in the print data of any image.
In addition, Japanese Patent Laid-Open No. H06-191041 (1994) describes a method in which the fixed mask pattern as shown in FIG. 2 is not used, but print data (“1”) of plural pixels to be printed continuously in a main scan direction or in a sub scan direction are distributed to be printed in different scans as many as possible.
FIG. 3 is diagrams showing an arrangement of print pixels of binary image data and the result in which the print pixels are distributed to two scans according to the method described in Japanese Patent Laid-Open No. H06-191041 (1994). In this way, the dots continuous in a main scan direction and in a sub scan direction are distributed equally to different scans. Thereby, not only image degradation due to variations in ejection characteristics of the printing element, but also defects such as ink overflow can be effectively reduced.
Even if the above multi-pass system is employed, under recent situations where a higher-quality printing is demanded, a density change or an uneven density due to a shift of a print position (registration) in a scan unit or in a nozzle line unit is seen newly as a problem. The shift of the print position in the scan unit or in the nozzle line unit is caused by fluctuations in distance between the print medium and the ejection opening surface (distance from a sheet), fluctuations of a conveying amount of the print medium or the like, and appears as a shift between planes of image printed in respective scans (or by respective nozzle lines).
For example, there will be considered a case where in an example shown in FIG. 3, a plane of dots (one circle) printed in the precedent scan and a plane of dots (double circle) printed in the subsequent scan are shifted by an amount corresponding to one pixel from each other in any one of a main scan direction and a sub scan direction. In this case, the dots (one circle) printed in the precedent scan and the dots (double circle) printed in the subsequent scan completely overlap to generate white areas on the print medium, and the white areas lower the density of image. Even in a case where the shift is not as large as one pixel, the fluctuations in the distance between the neighboring dots and the overlapped portion have a large impact on a coverage of dots to the white areas of the print medium, finally on the image density. Specifically, when the shift between the planes changes with the fluctuation in the distance between the print medium and the ejection opening surface (the distance from a sheet) or the fluctuation in the conveying amount of the print medium, the density of the uniform image also changes with these fluctuations, which results in being recognized as density unevenness.
Therefore, there is a demand for a method of producing print data in the multi-pass printing in which even if the print position shift occurs between the planes, the image quality is not remarkably deteriorated due to the position shift. In the present specification, regardless of fluctuations in any printing condition, a tolerance property that shows how hard to produce the density change or the uneven density due to the print position shift even if the print position shift between the planes occurs due to the fluctuation are called a “robustness”.
Japanese Patent Laid-Open No. 2000-103088 describes a method of producing print data for enhancing the above robustness. More specifically, this producing method has paid attention on a fact that the fluctuation in the image density due to the print position shift is, as described in detail in FIG. 3, caused by that binary print data distributed to plural times of scans or plural nozzle lines are completely complementary with each other. For reducing the extent of the above complementarity, the distribution of the image data to the plural times of the scans or the plural nozzle lines is carried out in a state of multi-valued data before binarizing and the multi-valued data after distributed are independently binarized.
FIG. 4 is a block diagram showing a control configuration example for realizing data distribution described in Japanese Patent Laid-Open No. 2000-103088. This figure shows an example of distributing print data to two print heads (two nozzle lines). Multi-valued image data received from a host computer 2001 are subject to various kinds of image processing (2004 to 2006), and thereafter, a multi value SMS section 2007 generates data for a first print head and data for a second print head based on the data that has been subjected to the various kinds of image processing. Specially the same multi-valued image data to which the image processing has been executed are prepared as the data for the first print head and the data for the second print head. In a first data conversion section 2008 and a second data conversion section 2009, conversion processing is executed using respective distribution coefficients. For example, a distribution coefficient of 0.55 is used to the data for the first print head and a distribution coefficient of 0.45 is used to the data for the second print head to execute the conversion processing. In consequence, the content of binarization processing to be executed later can be made different between the data for the first print head and the data for the second print head. Then, as described later in FIG. 5, overlaps of dots by the first print head and dots by the second print head finally formed can be generated in a certain ratio. It should be noted that Japanese Patent Laid-Open NO. 2000-103088 describes, in addition to an example where the distribution coefficient varies between the data for the first print head and the data for the second print head, an example where an error diffusion matrix used in error diffusion processing as binarization processing or threshold values in the error diffusion matrix varies.
The multivalued data converted as above are transferred to a first binarization processing section 2010 and a second binarization processing section 2011. In the first binarization processing section 2010 and the second binarization processing section 2011, the binarization processing is executed by an error diffusion method using an error diffusion matrix and threshold values, and the binarized image data are stored respectively in a first band memory 2012 and in a second band memory 2013. Thereafter, the first and second print heads eject ink according to the binary data stored in the respective band memories to perform printing.
FIG. 5 is a diagram showing an arrangement of dots on the print medium which are printed according to the aforementioned processing described in Japanese Patent Laid-Open No. 2000-103088. In FIG. 5, a black circle 21 shows a dot printed by the first print head, a white circle 22 shows a dot printed by the second print head, a circle 23 shown in a hatched line shows a dot printed in an overlapped manner by the first print head and the second print head.
Here, a case will be considered where in the same way as the example shown in FIG. 3, a plane of dots printed in the first print head and a plane of dots printed in the second print head are shifted by an amount corresponding to one pixel from each other in any one of a main scan direction or a sub scan direction. In this case, dots printed in an overlapped manner by both the first print head and the second print head are newly increased, but there exist also dots where the dot composed of two dots which are already printed in the overlapped manner is separated. Accordingly, when the determination is made based upon an area having a certain breadth, the coverage of the dots to the white area does not change so much, therefore not inviting a change of the image density. That is, the overlap of the dots is generated in a certain rate while basically eliminating complementarity or exclusiveness of dot formation by different scans or different print heads. Thereby, even if a shift of a print position due to fluctuations in scan speed of the carriage, fluctuations in distance (distance from a sheet) between the print medium and the ejection opening surface, fluctuations in conveying amount of the print medium, and the like is generated, the degree of fluctuations in image density or the density unevenness due to the fluctuations can be reduced to be small.
Further, Japanese Patent Laid-Open No. 2006-231736 describes the print data generation similar to that in Japanese Patent Laid-Open No. 2000-103088. Specifically, in the same way as Japanese Patent Laid-Open No. 2000-103088, the distribution coefficients are made different between the plural scans or between the plural printing element lines at the time of distributing the multi-valued image data to the plural scans or the plural printing element lines. In Japanese Patent Laid-Open No. 2006-231736, the distribution coefficient varies in accordance with pixel positions. For example, the distribution coefficients of two print heads vary in a linear way, in a periodical way, in a sinusoidal wave way, and in a combined wave way of a high frequency and a low frequency to the pixel position in a main scan direction, thereby restricting banding or color unevenness in the multi-pass printing.
However, in the print data generation method described in Japanese Patent Laid-Open No. 2000-103088 or Japanese Patent Laid-Open No. 2006-231736, since it is, as shown in an example in FIG. 4, necessary to perform quantization or gradation lowering process (binarization in an example in FIG. 4) for each of plurality of divided image data, there exists a problem that the processing load increases. More specifically, the quantization processing is executed using an error diffusion process or a dither process, but this processing itself has the processing circuit a processing load of which is relatively high, and therefore, in a case of executing quantization processing to each of the plural divided images, the processing load is further increased.
As a recent printing apparatus, there is provided a printing apparatus which has properties of high-quality and multi-color and further, a relatively wide printing width of 60 inches, for example, and further, high speeding of the print speed is in progress. For meeting demands for the high quality, the multi-color and the printing to a large scale of sheet, however, there occurs the problem that the image processing is complicated and a circuit scale or a memory capacity increases in proportion to the number of colors or print width and also the cost increases. In addition, high speed of the printing speed increases a load of the image processing and also increases costs. For example, in a case of storing the result of the image processing in a memory, transfer in a wide band is demanded due to many frequencies of data memory access and the circuit scale is increased for realizing the transfer in a wide band. Under these circumstances, performing the quantization to each divided image further causes an increase in the processing load. Therefore, it is preferable to reduce the load relating to the quantization to be as small as possible.